I agree I think teething plays a part. Atm my boy will let me brush his bottom no problem, but top ones he cries, I think his gums are a bit sore x
I think teething probably does play a part. She has one last bottom incisor to come through and that should be it until her second molars, so hopefully things will improve soon!
Is it possibly from teething? Particularly molars. My LB went through a phase of hating teeth brushing when his were on the move and it was only once they broke the surface that he went back to being happy to open his mouth for it. During the refusal time we basically decided that if we could get toothpaste in his mouth then we were happy, anything more was a bonus. You could try anbesol or other teething liquid/gel to numb the gums before brushing to see if that helps.